Method and system of lithography using masks having gray-tone features

ABSTRACT

A method forms patterns on a substrate by exposing the substrate a first time and exposing the substrate a second time using a mask containing gray-tone features. The gray-tone features locally adjust an exposure dose in regions corresponding to features defined in the primary exposure. Moreover, the gray-tone features enable the forming of features having different critical dimensions on a substrate. The gray-tone features may be sub-resolution features and formed by pixellation. The trim mask containing gray-tone features may have regions with different transmissivities.

CROSS-REFERENCE TO RELATED PROVISIONAL APPLICATION

[0001] The present patent application claims priority under 35 U.S.C.§119 from U.S. Provisional Patent Application Serial No. 60/361,612filed on Mar. 4, 2002. The entire contents of U.S. Provisional PatentApplication Serial No. 60/361,612 filed on Mar. 4, 2002 are herebyincorporated by reference.

FIELD OF THE PRESENT INVENTION

[0002] The present invention is directed to fabrication methods, such asdouble exposure lithography, which initially form a pattern on asubstrate and then trims the formed pattern. More particularly, thepresent invention is directed to a process and methodology ofcontrolling feature critical dimension using gray-tone mask features.

BACKGROUND OF THE PRESENT INVENTION

[0003] Conventional optical projection lithography has been the standardsilicon patterning technology for the past 20 years. It is an economicalprocess due to its inherently high throughput, thereby providing adesirable low cost per part or die produced. A considerableinfrastructure (including steppers, photomasks, resists, metrology, etc)has been built up around this technology.

[0004] In this process, a mask, or “reticle”, includes a semiconductorcircuit layout pattern typically formed of opaque chrome, on atransparent glass (typically SiO₂) substrate. A stepper includes a lightsource and optics/lenses that project light coming through the reticleand image the circuit pattern, typically with a 4× to 5× reductionfactor, on a photo-resist film formed on a silicon wafer. The termchrome refers to an opaque masking material that is typically but notalways comprised of chrome. The transmission of the opaque material mayalso vary such as in the case of an attenuating phase shift mask.

[0005]FIG. 1 is an example of a conventional optical projectionlithography apparatus. As illustrated in FIG. 1, the optical projectionlithography apparatus includes a light source 20, a photomask 22, andreduction optics 24. A wafer 26, having a layer of photo-resist 28thereon, is placed within the optical projection lithography apparatus,and the light-source 20 generates a beam of light 21 that is incidentupon the photomask 22. The reduction optics 24 projects the light beamto cause a pattern 30 that exposes the photo-resist layer 28, creatingthe pattern 30 of reacted material in the resist layer 28. In thismanner, a pattern 32, provided on the mask 22, is transferred to thephoto-resist layer 28 on the wafer 26.

[0006] The photo-resist pattern 30 is then transferred to the underlyingwafer 26 through standard etching processes using standard semiconductorfabrication techniques. Both positive and negative tone resists can beused to produce either positive or negative images of the mask patternon the wafer.

[0007] An example of a phase shift mask is illustrated in FIGS. 2 and 3.As illustrated in FIGS. 2 and 3, a dense-feature mask example 220 is aphase-shift mask comprising a grating pattern of periodic features. Itis noted that a dense grating pattern is only one example of adense-feature mask. FIGS. 2 and 3 are top and side views, respectively,of the phase-shift mask 220. The phase-shift mask 220 may be formed of,for example, fused SiO₂. Periodic trenches 23 are formed in the mask 220to provide an interference pattern upon illumination that results in thedesired photoresist pattern 30 on the wafer 26.

[0008] In the chromeless phase shift mask 220, periodic features ortrenches 23 are typically etched into the transparent mask material,which is typically quartz. The depth of these etched features 23 resultsin a relative phase difference in the illumination that is transmittedon either side of a phase boundary 36. When the relative phasedifference is 180 degrees, an interference null corresponding to thephase edge 36 is produced at the image plane, which is typically thewafer or substrate 26. It is noted that chromeless phase shift masksillustrated here are only one type of phase shift mask. There are manyother types of phase shift masks.

[0009] As the semiconductor industry continues to evolve and grow,feature sizes of the pattern are driven to an ever-smaller resolution.The driving force is the desire of these industries to remain on the“Moore's Law” growth curve. The “Moore's Law” growth curve calls for anexponential increase of circuit density versus production year that istypically accomplished by decreasing feature sizes. However, theresolution of an optical stepper is limited by the wavelength of thelight source, and is further limited by the numerical aperture (“NA”) ofthe lens.

[0010] The basic lithographic imaging relationships are:

[0011] 1) Resolution=k₁λ/NA; and

[0012] 2) Depth of Focus=k₂ λ/(NA)²;

[0013] where λ is the illumination wavelength, NA is the lens numericalaperture, and k₁ and k₂ are process constants.

[0014] In general, a shorter wavelength light source and/or a highernumerical aperture lens afford a higher-resolution system.State-of-the-art light sources provide a beam having a wavelength ofapproximately 193 nanometers. As stated above, the semiconductorindustry has been driving the need for critical feature sizes todecrease exponentially over time, while exposure light sourcewavelengths have only been decreasing linearly with time.

[0015] Carrying this scenario forward, current and future opticallithography will be required to image feature sizes of sub-wavelengthdimensions. Sub-wavelength optical lithography has been introduced withthe 180-nm Node device generation, fabricated using 248-nm opticallithography.

[0016] As noted above, the numerical aperture of the lens also drivesresolution. In this field, the cost of lenses having very high numericalapertures (“NA”) approaching 0.9 to 1.0 is very high. Moreover, linearNA increases are not sufficient to maintain pace with the need forexponentially decreasing feature sizes.

[0017] To meet this demand, Resolution-Enhanced optical lithographyTechnologies (“RET”) have become popular as techniques for providingpatterns with sub-wavelength resolution. These methods include off-axisillumination (“OAI”), optical proximity correction (“OPC”), andphase-shift masks (“PSMs”). Such resolution-enhanced optical lithographymethods are especially useful for generating physical devices on a waferthat require small size and tight design tolerance. Examples of suchphysical devices are the gate length of a transistor or the dimensionsof contact cuts formed in inter-layer dielectrics. However, theconventional RET methods face problems of layout complexity and datasize, mask fabrication complexity and resulting cost, and opticalproximity and spatial frequency effects which are discussed below.

[0018] In many circuit applications, it is an important designconstraint that the respective sizes of the narrow lines are consistentthroughout the circuit. For example, in a semiconductor device, thenarrow lines may form transistor gates, and it is important that thetransistor gates are similar in size so that the circuit has consistentand predictable gate delay values.

[0019] In general, in any optical lithography technique, the resultingoptical image intensity is a function of the proximity of features.Contrast is lost as feature pitch values decrease. As a result, theresulting size of features located in densely populated regions can bedifferent than the size for those features that are isolated from thedensely populated features. This is known as the “optical proximity”effect.

[0020] With respect to optical proximity effect, the critical dimensionof features depends on feature density. Moreover, optical proximityeffects can become more severe in sub-wavelength lithography. Theoptical proximity effects can result in dense lines 261 and an isolatedline 262 on wafer 26 being printed with different sizes, even if thesame size on the mask, as illustrated in FIG. 4, or dense contacts 263and an isolated contact 264 on wafer 26 being printed with differentsizes, even if the same size on the mask, as illustrated in FIG. 5.Since the performance of the circuit depends on the size and sizetolerance of the gates, this is an undesirable result.

[0021] Spatial frequency effects are caused by the “low-pass filter”behavior of a projection lithography lens wherein high spatialfrequencies do not pass through the lens. This results in cornerrounding and line end shortening. An example of this effect isillustrated in FIG. 6. As illustrated in FIG. 6, a desired image isrepresented by mask 2200, but the actual image pattern 265 on the waferis shortened and rounded.

[0022] To compensate for optical proximity and spatial frequencyeffects, additional features have been conventionally introduced on themask that can involve both printable as well as sub-resolution elements.In these methods, extra features such as serifs, mousebites,hammerheads, and scattering bars are added to the mask features in orderto correct for optical proximity effects and other spatial frequencyeffects. These conventional methods involve sophisticated algorithmswith very large data size, as different corrections are required foreach separation distance between the features. For this reason,conventional feature size correction (“OPC” or optical proximitycorrection) is a costly and time-consuming process.

[0023] Conventional OPC generally involves the processing of an enormousdata volume. The hierarchical data processing algorithms used forconventional circuit design are of limited utility because opticalproximity effects are based on the nature of geometries surrounding aparticular circuit element. For example, a 1X AND gate surrounded byregisters on all sides will perform differently than a 1X AND gatesurrounded by other 1X AND gates. Other examples of conventionallithography methods addressing the need for finer features orhigher-resolution features will be discussed below.

[0024] U.S. Pat. No. 5,415,835-B1 (“Brueck et al.”) discusses a methodof fine-line imaging based on laser interferometry. In Brueck et al.,dense gratings formed by laser interferometry are customized byadditional exposures using both interferometric and conventionallithography. Brueck et al. does not address optical proximity andspatial frequency effect problems thus limiting the ultimate density andflexibility of the patterns produced. In addition, the multipleexposures are not substantially independent in the optical sense due tothe resist's “memory” of previous exposure patterns. It is alsodifficult to make an arbitrary two-dimensional pattern in this way.

[0025] EP-0915384-A2 (“Suzuki et al.”) expands upon interferometriclithography. Suzuki et al. discloses using interferometricone-dimensional gratings to realize fine-line lithography together withsubsequent customization exposures using multiplex (sub-threshold)exposure doses. Suzuki et al. does not address optical proximity andspatial frequency effect problems thus limiting the ultimate density andflexibility of the patterns produced. The multiple exposures are notsubstantially optically independent due to the resist's “memory” of theprevious exposures. It is also difficult to realize an arbitrary 2Dpattern with this method. Since the fine features are only realized inone orientation, it is difficult to form patterns with fine features inboth the x & y directions.

[0026] WO-1/06320-A1 (“Levenson”) discloses re-usable “master” finefeature phase-shift masks that can be customized by multiple exposuremethods using conventional masks. Levenson discloses a “trade-offbetween a maximum density of features against the cost for low volumeruns”. Thus, the target application is primarily ASIC and thin-film headpatterns where the pattern density is not too great. Just as in theprevious Patents discussed above, this method does not mitigate opticalproximity and spatial frequency effects. It does not includesubstantially independent multiple exposures.

[0027] Finally, U.S. Pat. No. 6,184,151-B1 (“Adair et al.”) discloses amethod for forming square shape images wherein a first plurality oflines running in a first direction is defined in a first layer, and thena second resist is defined wherein the lines run in an intersectingpattern to those of the first layer, thereby creating sharp cornerswherever the first and second layers intersect and in open areas betweenthe lines. This process addresses the spatial frequency effect problemsof corner rounding and line-end shortening, but does not resolve theoptical proximity effect problem. The control of fine features throughpitch is important in order to realize the maximum pattern density andflexibility for applications.

[0028] One of the most common commercial implementations of phase shiftmask technology is the double exposure method. In this method, thecritical features are imaged using a phase shift mask and thenon-critical and trim features are imaged in a second exposure using aconventional chrome-on-glass mask.

[0029] An example of a double exposure phase shift method is illustratedin FIG. 7. Double exposure imaging has become an accepted method in thefield of resolution enhancement lithography.

[0030] In this method, fine features 42 are typically imaged on thesubstrate 26 in the first exposure, using a phase shift mask 31, anddefinition of other features 204 and trimming of undesired phase edgesare performed in a second exposure using a trim mask 38. The phase shiftmask may contain additional opaque features 203.

[0031] A typical double exposure phase shift method uses a conventionalchrome-on-glass binary photomask for the trim mask 38. In this case,chrome regions 40 on the trim mask 38 prevent desired features producedby the phase shift mask 31 from being exposed in the trim exposure.

[0032] Multiple critical dimensions are typically formed by varying thewidth of a chrome regulator structure 201 placed at each phase edge 36,as is illustrated in the mask of FIGS. 8 and 9.

[0033] More specifically, an alternating aperture phase shift mask,which has chrome regulators at each phase edge, is illustrated in FIGS.8 and 9. FIG. 8 is a top view and FIG. 9 is a cross sectional view.Unlike the chromeless phase shift mask, the alternating aperture phaseshift mask 122 has chrome regulators 201 placed at each phase edge 36.

[0034] Phase shift masks work by employing the principle of destructiveinterference of light to generate fine dark lines in photoresist. Aphase shift photomask is typically made of quartz (SiO₂) in whichfeatures are etched to a depth corresponding to a 180-degree phasedifference for the illumination light wavelength used. The equation fordetermining etch depth for optimum destructive interference is:

[0035] 1) d=λ/2(n−1)

[0036] where d is the etch depth, λ is the exposure wavelength and n isthe index of refraction of the glass mask at the exposure wavelength.

[0037] Upon illumination by the lithography apparatus as shown in FIG.1, each feature edge 36 forms a fine dark line in the photoresist 28.Note that since these dark line features correspond to phase boundaries36, these dark line features are topologically closed which has lead tothe development of double exposure phase shift methods in order to trimaway the undesired fine lines in a second exposure.

[0038] In this process, the critical features are imaged using a phaseshift mask and noncritical and trim features are imaged in a secondexposure using a conventional chrome-on-glass mask. One of thechallenges of this approach is the imaging of a variety of near minimumwidth feature sizes, by varying widths of chrome regulator features ateach phase transition.

[0039] As feature sizes continue to scale into the deep sub-wavelengthregime, it becomes more difficult to fabricate multiple sizes ofcritical dimension features by varying chrome regulator width. Inaddition, state of the art chromeless phase shift lithography methods,which are capable of the largest resolution enhancement, cannot be usedto image multiple fine feature critical dimensions in a single die.

[0040] Another example of a double exposure method is disclosed in U.S.Pat. No. 5,858,580 (Wang et al.). Wang et al. discloses creating a phaseshifting mask and a structure mask for shrinking integrated circuitdesigns. One disclosed embodiment includes using a two-mask process. Thefirst mask is a phase shift mask and the second mask is a single-phasestructure mask. The phase shift mask primarily defines regions requiringphase shifting. The single-phase structure mask primarily definesregions not requiring phase shifting. The single-phase structure maskalso prevents the erasure of the phase shifting regions and prevents thecreation of undesirable artifact regions that would otherwise be createdby the phase shift mask.

[0041] As feature sizes continue to move ever deeper into thesub-wavelength regime, it becomes increasingly difficult to image avariety of critical dimensions using this method. In addition,chromeless phase shift masks, which have great resolution enhancementpotential, cannot be used to image multiple critical dimensions.

[0042] The methods discussed above are also applicable to the case wherefine features are defined using interferometric processes or usingnanoimprint processes. In interferometric technology, a substrate isexposed interferometrically, using two or more coherent illuminationsources to generate an interference pattern on the substrate. Innanoimprint technology, a topographic pattern on a master template istransferred to a substrate by direct mechanical contact.

[0043] It is therefore desirable to develop a method that mitigatesoptical proximity and spatial frequency effects without adding complexoptical proximity correction features to the mask, while preserving theresolution enhancement aspects required by sub-wavelength lithography.This is especially desirable since conventional optical proximitycorrection approaches are becoming quite difficult to implement asimaging requirements continue to move deeper into the sub-wavelengthregime.

[0044] It is also desirable to eliminate basic optical proximityeffects, involving the defining or forming of fine lines in the x and ydirections through a variety of pitch values and to minimize spatialfrequency effects such as corner rounding and line-end shortening.

[0045] It is further desirable to simplify circuit layout and maskfabrication, resulting in lower cost and substantially decreased datavolume required for a typical design, thereby allowing for design ofstandard cells that can be accurately characterized independently oftheir eventual placement in a larger circuit.

[0046] Moreover, it is desirable to develop a method that incorporatessub-resolution features to allow for lithographic definition of featureswith locally variable critical dimension, which can be used in a varietyof lithographic processes, such as phase shift lithography,interferometric lithography, or nanoimprint technology.

[0047] It is also desirable to provide local control of the effectiveexposure dose that defines the critical dimension of the feature so thata wide variety of small features can be imaged without the need forchrome regulators or additional exposures.

[0048] Lastly, it is desirable to develop a method that incorporatessub-resolution features to allow for lithographic definition of featureswith locally variable critical dimension, which can be used incustomizing a master pattern on an individual substrate wherein themaster pattern was formed using a master template and nanoimprinttechnology.

SUMMARY OF THE PRESENT INVENTION

[0049] One aspect of the present invention is a method of forming apattern on a substrate. The method forms a pattern on a substrate andexposes the substrate using a mask containing gray-tone features.

[0050] Another aspect of the present invention is a method of designinga mask in which a primary exposure is assumed. The method placesgray-tone features on a layout of the mask to locally adjust an exposuredose in regions corresponding to features defined in the primaryexposure and places other features on the layout of the mask.

[0051] A further aspect of the present invention is a trim mask havinggray-tone features.

[0052] A fourth aspect of the present invention is a method of forming afeature having a critical dimension on a substrate. The method exposesthe substrate using a mask containing gray-tone features.

[0053] A fifth aspect of the present invention is a mask set for aprocess for providing patterns on a substrate. The mask set includes afine feature mask containing a pattern of dense features and a trim maskcontaining gray-tone features to produce multiple trimmed patterns offine features.

[0054] A sixth aspect of the present invention is a method of forming arandom contact array on a substrate. The method exposes the substrate toprovide a pattern of dense contact features of a predetermined pitch andcritical dimension and exposes the substrate with a trim mask containinggray-tone features to provide multiple trimmed patterns on thesubstrate, the trimmed patterns including both densely populated andsparsely populated regions of features, the critical dimension of thefeatures in the densely populated regions and sparsely populated regionsbeing substantially independent of feature density.

[0055] A seventh aspect of the present invention is a mask set for aprocess for providing patterns on a substrate. The mask set includes afine feature mask and a trim mask containing gray-tone features toproduce multiple trimmed patterns of fine features.

[0056] An eighth aspect of the present invention is a method of formingpatterns on a substrate. The method produces fine features usingnanoimprint methods and exposes the substrate using a mask containinggray-tone features.

[0057] A ninth aspect of the present invention is a computer aideddesign method for designing a mask. The method specifies, through aninput of a user, a geometric property of a desired substrate feature anddetermines automatically, based upon the user specified geometricproperty of the desired substrate feature, mask features for a gray-tonemask.

[0058] A tenth aspect of the present invention is a method of forming apattern on a substrate. The method imprints a pattern on a substrate andexposes the substrate to change the imprinted pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment or embodiments and are not to be construed as limiting thepresent invention, wherein:

[0060]FIG. 1 is a schematic of a prior art optical projectionlithography apparatus;

[0061]FIG. 2 is a top view schematic of a chromeless phase shift mask;

[0062]FIG. 3 is a cross sectional view of a chromeless phase shift mask;

[0063]FIG. 4 is an illustration of an optical proximity effect withrespect to fabricating lines;

[0064]FIG. 5 is an illustration of an optical proximity effect withrespect to fabricating contact holes or pillars;

[0065]FIG. 6 is an illustration of a spatial frequency effect withrespect to fabricating lines;

[0066]FIG. 7 is a schematic illustration of a prior art double exposurelithography approach;

[0067]FIG. 8 is a top view schematic of prior art use of chromeregulators to locally modify the critical dimension;

[0068]FIG. 9 is a cross sectional view of prior art use of chromeregulators to locally modify the critical dimension;

[0069] FIGS. 10-13 are top views of various dense-feature mask patternconfigurations in accordance with the present invention;

[0070]FIG. 14 is a top view of a wafer exposed by the dense feature maskof FIG. 10 in accordance with the present invention;

[0071]FIG. 15 is a graphical flow diagram illustrating top views of awafer undergoing a trimming process in accordance with the presentinvention;

[0072]FIG. 16 is a graphical flow diagram illustrating top views of awafer undergoing an interconnect process in accordance with the presentinvention;

[0073]FIG. 17 is a top view of trimmed fine features formed according tothe technique of the present invention illustrating the absence ofoptical proximity effects;

[0074]FIG. 18 is a top view of a dense feature mask including bothprintable features and sub-resolution features in accordance with thepresent invention;

[0075]FIG. 19 is a graphical flow diagram illustrating a trimmingoperation performed on a wafer including solid patterns formed byexposure of the sub-resolution features of the dense feature mask ofFIG. 15; and

[0076]FIG. 20 is a graphical flow diagram illustrating the extension ofthis method to produce sets of fine features with different orientationsand position offsets;

[0077]FIG. 21 is a top view schematic of one embodiment of a gray-tonetrim mask according to the concepts of the present invention;

[0078]FIG. 22 is a schematic of a double exposure phase shift processusing the gray-tone trim mask of FIG. 21 to locally modify the criticaldimension, according to the concepts of the present invention;

[0079]FIG. 23 is a top view schematic of another embodiment of agray-tone trim mask according to the concepts of the present invention;and

[0080]FIG. 24 is a schematic of a double exposure phase shift processusing the gray-tone trim mask of FIG. 23 to locally modify the criticaldimension, according to the concepts of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0081] The present invention is directed to an imaging approach thatovercomes the limitations of the conventional techniques, and confers anumber of advantages. It addresses the problems of optical proximity andspatial frequency effects while maintaining the resolution-enhancementperformance required by sub-wavelength lithography. Moreover, thepresent invention addresses the problems associated withlithographically defining features with locally variable criticaldimension. The present invention also provides local control of theeffective exposure dose that defines the critical dimension of thefeature so that a wide variety of small features can be imaged withoutthe need for chrome regulators or additional exposures.

[0082] In the following description, the phrase, “lines,” refers toeither the trenches or the raised areas; e.g., plateaus; on a wafer.Moreover, the phrase, “contacts,” refers to either the holes or pillarson a wafer. The described photoresists may either be a negative tone ora positive tone. The descriptions are applicable to either positive ornegative imaging of the wafer or substrate.

[0083] With respect to spatial frequency effects and optical proximityeffects, any image that is lithographically exposed can be thought of inFourier space, where components of various spatial frequencies sum toform the complete image. The lens acts as a low-pass filter because ithas a finite aperture. Spatial frequency effects cause corner roundingand line-end shortening because higher diffraction orders are filteredout. Optical proximity effects cause the same features spatially apartfrom each other on a substrate to realize a size differential eventhough these features were formed using the same mask feature size. Thiseffect is typically described quantitatively in terms of criticaldimension versus pitch.

[0084] Lastly, the phrase, “dense features,” refers to an area on thesubstrate having a multitude of features positioned very closely to eachother.

[0085] In describing the concepts of the present invention below,examples primarily directed to double exposure photolithography havebeen used. However, it is noted that the various below describedconcepts of the present invention are also applicable to the case wherefine features are defined using interferometric processes or usingnanoimprint processes. In interferometric technology, a substrate isexposed interferometrically, using two or more coherent illuminationsources to generate an interference pattern on the substrate. Innanoimprint technology, a topographic pattern on a master template istransferred to a substrate by direct mechanical contact.

[0086] In the present invention, a mask is provided including a denserepetitive structure of features that results in a large array ofdensely populated features on the film or substrate. The pattern ofdense features may be locally or globally periodic. The mask is designedto print dense features near the resolution limit of the lithographystepper used, thus defining a pattern “grid.” The minimum width features(such as transistor gates and contacts) are laid out on this grid. Theallowed feature grid locations are integer multiples of the minimum gridpitch.

[0087] Using this mask, the substrate is exposed to provide a pattern ofregular dense features of a predetermined pitch and critical dimension.Following this first dense feature formation exposure, a trimmingexposure is performed to remove any unwanted pattern features.

[0088] It is noted that a pattern of regular dense features of apredetermined pitch and critical dimension can be formed usinginterferometric lithography, and a trimming exposure is performed,thereafter, to remove any unwanted pattern features.

[0089] It is further noted that the present invention may utilize atemplate and nanoimprinting to create a pattern of dense features havinga predetermined pitch and critical dimension, the pattern of densefeatures may be locally or globally periodic. In this embodiment, atrimming exposure is performed to remove any unwanted pattern features.

[0090] Optionally, an additional exposure is performed, adding furtherfeatures as well as interconnecting the previously formed features toform a circuit. This exposure is substantially independent of theprevious exposures, thus minimizing effects such as spatial frequencyeffects (corner rounding and line end shortening).

[0091] For example, when using less general dense fine feature masks,only two exposures are necessary: dense-grating and trim. On the otherhand if a simple one-dimensional grating mask is used, three exposuresare required: dense-grating, trim, and interconnect. In addition to thegrid layout restriction, this particular embodiment also requires theminimum width features to be oriented in the same direction.

[0092] In the present invention, all small features are generated usingthe exact same density optical image patterns; and therefore, maximumconsistency between features is established. The small features maycomprise gates of transistors or contact holes. Small features are thusproduced without the proximity effects realized by the conventionaltechniques using simple, reusable dense-grating masks. Customized andless expensive trimming masks can then be used to complete the desiredpattern as well as interconnect the circuit components.

[0093] In this manner, only a single dense-grating is required forgenerating any of a number of different circuits and patterns. There-usability of the dense-grating mask is desirable since this is oftenthe most difficult and expensive mask to fabricate. This is especiallytrue if the dense grating mask is a phase-shift mask. A phase-shift maskis capable of imaging dense features very close to the Rayleigh limitfor optical projection steppers. (Pitch_(MIN)=0.5λ/(NA)). Dense regularpatterns also are routinely obtained using interference lithography.

[0094] As noted above, FIG. 1 is a schematic block diagram of aconventional optical projection lithography apparatus. The conventionaloptical projection lithography apparatus includes a light source 20, aphotomask 22, and reduction optics 24. A wafer 26 having a layer ofphotoresist 28 is presented to the conventional optical projectionlithography apparatus, and the light-source 20 generates a beam of light21 that is incident upon the photomask 22 and reduced by reductionoptics 24 to cause a pattern 30 to be exposed in the photoresist layer28. In this manner, a pattern 32 provided on the mask 22 is transferredto the photoresist layer 28 on the wafer 26.

[0095] In a preferred embodiment of the present invention, thedense-feature mask 220 is a phase-shift mask comprising a gratingpattern of periodic features. FIGS. 2 and 3 are top and side views,respectively, of a phase-shift mask that is a preferred embodiment ofmask 220 used with respect to the present invention. The phase-shiftmask 220 may be formed of, for example, fused SiO₂. Periodic trenches 23are formed in the mask 220 to provide an interference pattern uponillumination that results in the desired photoresist pattern 30 on thewafer 26.

[0096] Although a simple one-dimensional mask grating is shown in FIGS.2 and 3, the present invention is applicable to phase-shift masks of avariety of patterns. The present invention is also applicable to othertypes of phase-shift masks such as alternating aperture (AAPSM) orattenuating phase-shifters (APSM).

[0097] A more detailed discussion of these phase-shift masks is setforth in co-pending U.S. patent application Ser. No. 09/952,185, filedon Sep. 13, 2001, entitled “Method Of Design And Fabrication OfIntegrated Circuits Using Regular Arrays And Gratings.” The entirecontents of U.S. patent application Ser. No. 09/952,185, filed on Sep.13, 2001, are hereby incorporated by reference.

[0098] With reference to FIGS. 10-13, the line features 23A, 23B, and23C can be formed in a variety of configurations. In the configurationof FIG. 10, horizontal line features 23A of mask 2203 are formedparallel to each other in the X-direction, while in FIG. 11, verticalline features 23B of mask 2204 are formed parallel to each other in theY-direction. The mask shown in FIG. 12 includes features formed in ahorizontal orientation 23A in a first region of the mask 2205 andfeatures formed in a vertical orientation 23B in a second region of themask 2206.

[0099] Note, however, that in alternative embodiments, the features maybe formed in other patterns, including locally regular patterns. Forexample, in the mask of FIG. 13, a unique feature pattern 23C isemployed. Other such unique combinations of patterns are applicable tothe present invention.

[0100]FIG. 14 is a top view of the resulting pattern formed on the wafer26, assuming exposure by the dense feature mask 2203 of FIG. 10. Aplurality of periodic thin lines 34 of photoresist material is formed onthe wafer 26 (in the case of a positive resist). A preferred embodimentof the present invention produces these grating features with aphase-shift mask; however it is noted that the present invention mayalso use interferometric or imprint definition processes to producethese grating features.

[0101] As noted above, the present invention is directed to fabricatingphysical structures on a substrate or wafer. FIGS. 15 and 16 graphicallyillustrate a process for forming these structures according to theconcepts of the present invention.

[0102] As shown in FIG. 15, a wafer or substrate 26 is exposed using agrating 31 which including a line pattern 34 to provide a pattern ofregular dense lines or features of a predetermined pitch and criticaldimension on the substrate 26. Thereafter, a trim mask 38, which mayinclude gray-tone features, is exposed on the wafer 26.

[0103] It is noted that a pattern of regular dense lines or features ofa predetermined pitch and critical dimension on substrate 26 can beformed using interferometric lithography, and a trimming exposure, usingtrim mask 38 that may include gray-tone features, is performed,thereafter, on the wafer 26 to remove any unwanted pattern features.

[0104] It is further noted that the present invention may utilize atemplate and nanoimprinting to create a pattern of dense lines orfeatures having a predetermined pitch and critical dimension onsubstrate 26, the pattern of dense features may be locally or globallyperiodic. In this embodiment, a trimming exposure, using trim mask 38that may include gray-tone features, is performed on the wafer 26 toremove any unwanted pattern features or to customize the wafer 26.

[0105] For example, in the case of transistors having a consistent,narrow gate length, the gate length of each transistor corresponding tothe width of each line, trim features 40 are formed on the customizedtrim mask 38 and exposed on the wafer 26 at corresponding locations 41to form a resulting trimmed pattern 42, comprising a plurality oftransistors of equal gate length along their critical dimension. In thismanner, a standardized and relatively inexpensive dense fine featuremask can be used to form the critical dimension of the transistor lengthunder tight tolerance conditions.

[0106] This is combined with the customized trim mask 38, which maycontain gray-tone features, of relatively loose tolerance constraints toprovide a trimmed pattern 42 on the wafer 26. A certain degree ofmisalignment can be tolerated on the trim mask 38 along with oversizeand undersize error in the trim features 40, as well as exposure error.These loose tolerance constraints are acceptable because of the oftenrelatively wide pitch, or distance, between centers of the narrowfeature lines 34 and loose tolerance constraints are typical of trimexposures. Additionally, the critical dimension, width, of the featurelines 42 is not determined by the trim-mask features 40. It is generallyeasier to erase existing features than to accurately place new features.Thus, the present invention is especially amenable to applicationsinvolving a dense array of features at a minimal critical dimension.

[0107] Following formation of the trimmed pattern 42 on the wafer 26,the exposure of the trimmed pattern 42 is developed according tostandard semiconductor chemical processes that alter the chemistry inthe photo-resist. Following this, the wafer is subjected to a “hardbake” such that the photoresist is no longer susceptible to light. Thewafer is then re-coated with photoresist in preparation for the patterninterconnection process.

[0108] With reference to FIG. 16, following formation of the trimmed anddeveloped trim patterns 42 on the wafer 26, the fresh layer ofphotoresist on the wafer 26 is next exposed by an interconnect mask 44including interconnect features 46. In this manner, assuming the trimmedpattern 42 represents transistor gates, the interconnect features 46provide a means for interconnecting the gates 42 so as to form acircuit. Again, in this case, the interconnect mask 44 iscustom-designed for the particular circuit to be interconnected.

[0109] Since a standard dense feature mask is used to generate patternsof fine lines with a given critical dimension in the circuit, thepresent invention is immune to optical proximity effects. For example,in the pattern of trimmed features 48 shown in FIG. 17, patterns 49Aformed in densely populated region 50 of the pattern of trimmed features48 and patterns 49B formed in a sparsely populated region 52 of thepattern of trimmed features 48 remote to the densely populated region 50have identical feature sizes.

[0110] However, assuming the original dense feature mask has morecomplex pattern, such as the “L”-shaped pattern 23C of FIG. 13, as thesefeatures become small, sharp features, such as corners and line-endstend to become distorted. For this reason, various forms of opticalproximity correction or spatial frequency correction features may beconfigured directly into the dense feature mask template in order topre-distort the template pattern. For example, well-known techniquessuch as hammer-heads, serifs, and mouse bites may be added to the cornerfeatures so that the exposed final feature resembles the desiredfeature. By incorporating the optical proximity correction and/orspatial frequency correction directly in the dense feature mask, thestandard features are automatically corrected on the dense feature maskrather than on the design-specific trim mask and interconnect mask,thereby providing an economically efficient solution to the opticalproximity correction issue.

[0111] With reference to FIG. 18, the present invention is also amenableto use with a dense feature mask 227 having sub-resolution gratings 60.

[0112] As illustrated in FIG. 19, printable features 58 of the densefeature mask 227 of FIG. 18, when exposed, result in fine featuresformed on the wafer 26, while the sub-resolution features 60 of thephase-shift mask 227 result in solid patterns on the wafer 26. Throughthe use of a trim mask 38, which may contain gray-tone features, thesesolid patterns can be trimmed appropriately so as to form interconnects62 between the trimmed fine features 64 as shown.

[0113] It is noted that a pattern of regular features and solid patternson substrate 26 can be formed using interferometric lithography, and atrimming exposure, using trim mask 38 that may include gray-tonefeatures, is performed, thereafter, on the wafer 26 to remove anyunwanted pattern features.

[0114] It is further noted that the present invention may utilize atemplate and nanoimprinting to create a pattern of regular features andsolid patterns on substrate 26. In this embodiment, a trimming exposure,using trim mask 38 that may include gray-tone features, is performed onthe wafer 26 to remove any unwanted pattern features or to customize thewafer 26.

[0115] Since the imaging of each set of exposures, in the presentinvention, is independent of the previous exposures, the method of thepresent invention can be used to produce fine features such astransistor gates with multiple orientations and/or offsets. This isillustrated in FIG. 20.

[0116] With respect to producing fine features with multipleorientations and/or offsets, first the approach illustrated in FIG. 15is used to create fine features in one orientation. More specifically, asubstrate is photolithographically exposed with a dense-grating mask toprovide a first exposed pattern on the substrate. The substrate is thenphotolithographically exposed with a trim mask to trim the first exposedpattern to provide multiple trimmed patterns 42 on the substrate 26 asillustrated in FIG. 20.

[0117] Next, the substrate 26 is recoated with resist andphotolithographically exposed with a dense-grating mask 229 with anotherorientation or offset 23B. Then, the substrate is photolithographicallyexposed with a trim mask 38B with pattern 40B to trim this second densegrating pattern to provide a second set of multiple trimmed patterns 68on wafer 26. Thus, two sets of multiple trimmed patterns are formed 42and 68 each having its own orientation and offset. As an alternative tothe second resist coat, a hardmask or double bilayer resist process canalso be used in the same manner as described below with respect to FIGS.16 and 17.

[0118] With respect to the processes illustrated in FIGS. 16 and 17, asubstrate is first photolithographically exposed with a dense-gratingmask to provide a first exposed pattern on the substrate. The substrateis next photolithographically exposed with a trim mask, which maycontain gray-tone features, to trim the first exposed pattern to providemultiple trimmed patterns on the substrate. Next, the substrate isrecoated with resist and photolithographically exposed with aninterconnect mask to provide an interconnect pattern on the trimmedpattern for interconnecting a subset of the multiple trimmed patterns.

[0119] As an alternative to the second resist coat, a hardmask or doublebilayer resist process can also be used. In a hardmask process, anadditional masking layer, typically silicon nitride or silicon dioxide,is formed on the wafer prior to resist coating. This layer is definedand etched to form a mask that can be used in processing of theunderlying substrate. In a double bilayer process, two layers of resistare formed such that patterns formed in the top layer of resist are usedto locally control the removal of the bottom layer of resist such that apattern formed in the bottom layer of resist corresponds to thegeometric union of multiple exposures.

[0120] A less general dense-feature pattern can also be used whicheliminates the need for a third exposure.

[0121] According to the concepts of the present invention,lithographically exposing the substrate with a dense feature maskprovides a first exposed pattern having a predetermined criticaldimension. In this case, following photolithographically exposing thesubstrate again with the trim mask, which may contain gray-tonefeatures, the multiple trimmed patterns have substantially the samecritical dimension.

[0122] A first subset of the multiple trimmed patterns may be located ina sparsely populated region of the substrate, and a second subset of themultiple trimmed patterns may be located in a densely populated regionof the substrate. The first subset and second subset of patterns havesubstantially the same critical dimension. This is illustrated in FIG.15 (42) and FIG. 17 (49A and 49B). The multiple trimmed patterns maycomprise transistors wherein the critical dimension represents thelength of the gate length of each transistor.

[0123] The substrate may comprise a semiconductor substrate having afirst photo-resist layer, wherein the multiple trimmed patterns areprovided in the first photoresist layer, and wherein followingphotolithographically exposing the substrate with the trim mask, thefirst photoresist layer is developed. Following developing of the firstphotoresist layer, a second photoresist layer may be formed over thedeveloped first photoresist layer, in which case, the interconnectpattern is provided in the second photoresist layer.

[0124] Another embodiment uses a less general dense-feature pattern onthe photomask. This pattern is then “customized” by the second trimexposure into a desired circuit pattern. This embodiment only requirestwo exposures and does not require a second resist coat. This embodimentis illustrated in FIG. 19.

[0125] Other possible embodiments involve the triple-exposure methoddescribed above with the second resist replaced by either a hardmask ordouble bilayer resist process.

[0126] The dense-pattern features can be generated by either photomasksor other imaging/patterning methods not necessary produced by opticalmask projection methods. Such methods may include laser interferometryor nano-imprint methods. When photomasks are used, a preferredembodiment utilizes phase-shift masks to generate the dense-featurepatterns.

[0127] In interference lithography, fine features are centered onregions of destructive interference; i.e., nulls. In phase-shiftlithography, these nulls correspond to the boundary of a particulartransmitting region on a mask, and a second transmitting region with aphase-shift typically of about 180 degrees. In some applications, phaseshift masks with phase shifts between zero and ±180 degrees may be used.

[0128] An attenuating phase shift mask (APSM) is a phase shift mask inwhich there are multiple regions. A set of regions is included for whichthere is a relative phase shift of zero degrees and a transmissivitybetween zero and one. Other sets of regions have a phase shift typicallyof about 180 degrees and a transmissivity between zero and one. In someapplications, attenuating phase shift masks with phase shifts betweenzero and ±180 degrees may be used.

[0129] In laser interferometry, these nulls correspond to regions ofdestructive interference of two wavefronts. The critical dimension inresist of interferometrically-defined features (phase-shift orotherwise) is determined primarily by the exposure dose. Thus, for asingle exposure in which all features receive the same dose, onecritical dimension is defined at each null. Conventionally forming thesecritical dimensions is not addressed during a single phase-shiftexposure. It would be beneficial for multiple fine line criticaldimensions to be defined in resist during a single phase-shift orinterferometric exposure.

[0130] In the present invention, multiple fine line critical dimensionsare defined in resist during a single feature definition exposure byadding exposure of a gray-tone mask to locally partially exposedifferent regions of a substrate, thereby allowing for a range of fineline critical dimensions to be defined by the feature definitionexposure. This gray-tone mask can be implemented as a mask with varyingdegrees of transmission or as a pixellated mask with features below thepitch resolution of the stepper that produce a similar dose variationeffect. The gray-tone mask exposure may either be the trim exposure orbe in addition to the trim exposure of a double exposure PSM process.

[0131] To simplify the gray scale mask fabrication process, the stepperresolution can be intentionally worsened or degraded during the grayscale exposure by a detuning method. For example, the detuning can berealized by the partial coherence of the exposure being decreased or thenumerical aperture of the lens (NA) being decreased. This detuningpermits a larger pixellation pitch to be used on the mask, thusdecreasing the mask write time and resolution requirements.

[0132] The two most difficult patterning levels in semiconductorprocessing are transistor gates and contact holes. Attenuatingphase-shift masks have been conventionally used to increase theresolution and process latitude for leading edge applications requiringsmall contact holes. The transmission of these attenuated phase-shiftmasks has been limited to around 6% due to the problem of sidelobeprinting at larger transmission values. In addition, the correction ofoptical proximity effects is difficult for leading edge contact patternswith complex ternary masks sometimes being used. In addition toconsiderably increasing mask fabrication complexity, the conventionalapproaches do not allow for maximum packing density due to the area theyconsume. An example of a conventional method uses a darkfield mask tonewith binary masks with sub-resolution OPC features.

[0133] The present invention uses “dense-only” attenuated phase-shiftmasks to fabricate random contact patterns. The method, according to theconcepts of the present invention, involves the use of two exposures.The first exposure uses a dense pillar array, using a brightfieldattenuated phase-shift mask, which images a dense array of holes using anegative resist. The brightfield mask tone used is important forimproving the process latitude. The second exposure trims out theundesired contact features. The sum of these two exposures produces arandom array of holes.

[0134] Since only dense contact features are imaged, proximity andsidelobe effects are eliminated. Thus, the use of complex ternary or OPCcontact masks is not required. Since sidelobe effects are absent, theuse of high transmission attenuated phase-shift masks is possible,greatly improving lithographic performance. In fact, “chromeless”attenuated phase-shift masks can be used where the transmission of thepatterned regions is 100%. The present invention uses a brightfield masktone together with high transmission attenuated phase-shift mask andfull exposure doses for each level.

[0135] The present invention is also directed to a design methodology.In this methodology, according to the present invention, standardcircuit subcell designs are designed or laid out wherein all finefeatures lie on a regular pattern. The standard circuit subcell designsare compatible with a method of forming patterns on a substrate byexposing the substrate to provide a pattern of regular dense features ofa predetermined pitch and critical dimension and exposing the substrateto provide multiple trimmed patterns on the substrate, the trimmedpatterns including both densely populated and sparsely populated regionsof features, the critical dimension of the features in the denselypopulated regions and sparsely populated regions being substantiallyindependent of feature density.

[0136] In another embodiment, the methodology designs standard circuitsubcells wherein all fine features lie on a regular pattern, and thestandard circuit subcell designs are compatible with a mask set forproviding patterns on a substrate having a fine feature mask containinga pattern of regular dense features and a trim mask for producingmultiple trimmed patterns of fine features.

[0137] The present invention also contemplates a computer-aided ornon-computer aided design methodology. This methodology assumes a firsttemplate of dense-only features as a first mask level and places a firstset of trim features on a second mask level to coincide with selectedfeatures on the first template, such that a first subset or superset offeatures desired for fabrication correspond to a geometric function ofthe first template and the first set of trim features. Thecomputer-aided methodology can further place additional features onadditional mask levels, such that features desired for fabricationcorrespond to a geometric function of the first subset or superset offeatures desired for fabrication and the additional features, thegeometric function being determined by a destination lithographicprocess in which the imaging of the first subset or superset of featuresdesired for fabrication of the trim features and the imaging of theadditional features are substantially independent. The placing ofadditional features can be defined by assuming a second template ofdense-only features as a first mask level and placing a second set oftrim features on a second mask level to coincide with selected featureson the second template, a second subset or superset of features desiredfor fabrication corresponding to a geometric function of the secondtemplate and the second set of trim features; or by placing additionalfeatures on additional mask levels, such that features desired forfabrication correspond to a geometric function of the second subset orsuperset of features desired for fabrication and the additionalfeatures, the geometric function being determined by a destinationlithographic process in which the imaging of the second subset orsuperset of features desired for fabrication and the imaging of theadditional features are substantially independent.

[0138] In a further embodiment of the present invention, the presentinvention extends the capability of double exposure phase shiftlithography. It enables the imaging of multiple fine feature criticaldimensions by using gray-tone mask features on either the phase shift ortrim masks. This permits the use of chromeless phase shift orinterferometric fine feature imaging for a much wider range ofapplications.

[0139] As noted above, a chromeless phase shift mask 220 is shownschematically in FIGS. 2 and 3. In this chromeless phase shift mask 220,features 23 are typically etched into the transparent mask material,which is typically quartz. The depth of these etched features 23 resultsin a relative phase difference in the illumination that is transmittedon either side of a phase boundary 36. When the relative phasedifference is 180 degrees, an interference null corresponding to thephase edge 36 is produced at the image plane, which is typically thesubstrate 26.

[0140] There are several other varieties of phase shift masks. Analternating aperture phase shift mask, which has chrome regulators ateach phase edge, is illustrated in FIGS. 8 and 9. FIG. 8 is a top viewand FIG. 9 is a cross sectional view of the alternating aperture phaseshift mask, which has chrome regulators at each phase edge. Like thechromeless phase shift mask 220, the alternating aperture phase shiftmask 122 has chrome regulators 201 placed at each phase edge 36.

[0141] Phase shift masks work by employing the principle of destructiveinterference of light to generate fine dark lines in photoresist. Aphase shift photomask is typically made of quartz (SiO₂) in whichfeatures are etched to a depth corresponding to a 180 degree phasedifference for the illumination light wavelength used. The equation fordestructive interference is:

2)d=λ/2(n−1)

[0142] where d is the etch depth, λ is the exposure wavelength and n isthe index of refraction of the glass mask at the exposure wavelength.Upon illumination by the lithography apparatus shown in FIG. 1, eachfeature edge 36 forms a fine dark line in the photoresist 28.

[0143] Further as noted above, since these dark line features correspondto phase boundaries 36, dark line features are topologically closed,which has lead to the development of double exposure phase shift methodsin order to trim away the undesired fine lines in a second exposure.

[0144] Double exposure imaging has become an accepted method in thefield of resolution enhancement lithography. In this method, finefeatures are typically imaged on a substrate in the first exposure,using a phase shift mask, and other features as well as trimming ofundesired phase edges is performed in a second exposure using a trimmask. The phase shift mask may contain additional opaque features. Thephase shift mask may be a strong phase mask or a weak phase mask. Astrong phase shift mask is a phase shift mask for which ideal denseperiodic grating patterns generate no zero-order diffracted light. Aweak phase shift mask is a phase shift mask for which ideal denseperiodic grating patterns generate some zero-order diffracted light.

[0145] As noted above, conventional processes use a chrome-on-glassbinary photomask for this trim mask. In this case, chrome regions on thetrim mask prevent desired features produced by the phase shift mask frombeing exposed in the trim exposure.

[0146] Multiple critical dimensions are typically formed by varying thewidth of a chrome regulator structure placed at each phase edge. Asfeature sizes continue to move ever deeper into the sub-wavelengthregime, it becomes increasingly difficult to image a variety of criticaldimensions using this method. In addition, chromeless phase shift masks,which have the greatest resolution enhancement potential, cannot be usedto image multiple critical dimensions. A similar problem is encounteredwhen interferometric lithography is used.

[0147] This embodiment of the present invention places gray-tonefeatures on either the trim or phase shift mask of a double exposureprocess. A gray-tone mask feature allows zero order light to pass butcontains no modulation information (higher order diffractioncomponents). Mathematically, this type of feature has a constraint onthe pitch given by: P <λ/NA(1+σ).

[0148] In this expression, λ is the exposure wavelength, NA is thelithography lens numerical aperture and σ is the lithographyillumination partial coherence. Note that modem lithographic exposuretools feature variable NA and σ which allow “detuning” of the projectionoptics in order to relax the minimum pitch requirement. This capabilityis useful to ease the mask making requirements for gray-tone features. Agiven gray-tone intensity can be achieved by a variety of feature shapesand placement algorithms satisfying the pitch constraint above.

[0149] An example of this embodiment of the present invention isillustrated schematically in FIGS. 21 and 22. FIG. 21 shows an exampleof one embodiment of a gray-tone trim mask 210. This gray-tone mask 210includes solid opaque features 211, and sub-resolution features 212 and213 of various widths and spacings. For this example, the sub-resolutionfeatures 212 have a pitch, P1, and the sub-resolution features 213 havea pitch, P2. This type of gray-tone mask may be referred to as apixellated gray-tone mask.

[0150]FIG. 22 shows a double exposure lithography process using thepixellated gray-tone trim mask 210 for the trim exposure. In thisexample, a chromeless phase shift mask 225 is used for imaging the finefeatures or dense fine features. The trim mask 210 is designed so thatthe pixellated gray-tone regions 211, 212, and 213 have positions thatare a function of the phase edge position 36. These pixellated gray-toneregions 211, 212, and 213 will prevent the fine features or dense finefeatures corresponding to the interference null at phase edge 36 frombeing removed in the trim exposure.

[0151] The variation in transmissivity results in local variation ofcritical dimension. For example, the solid opaque region on trim mask210 results in a feature on the substrate 26 with critical dimension230. The region 212 on trim mask 210 with pitch, P1, results in afeature on the substrate 26 with critical dimension 231. The region 213on trim mask 210 with pitch, P2, results in a feature on the substrate26 with critical dimension 232. By varying the gray-tone transmissionvalue, a wide range of near minimum width critical dimension featurescan be imaged in a single die.

[0152] In this embodiment, sub-resolution pixels of various widths andspacings are used to produce gray-tones. The gray-tone regions arepositioned in locations corresponding to those features that are to bepreserved in the trim exposure. These gray-tone features may be formedusing pixellation.

[0153] Another embodiment of the present invention is shownschematically in FIGS. 23 and 24. This embodiment of the presentinvention uses continuously variable mask transmission to implement thegray-tone function. This is in place of the mask pixellation utilized inthe previous embodiments discussed above. Such types of masks areavailable for example under the “HEBS” (high energy beam sensitiveglass) name.

[0154] An example of a variable transmission mask is shown in FIG. 23.This trim mask 2100 has three non-background regions 211, 221, and 222that have different transmissivities.

[0155]FIG. 24 shows this mask applied to a double exposure process. Notethat the regions 211, 221, and 222 on the trim mask 2100 when combinedwith the phase shift mask 2201 in a double exposure, result in features230, 231, and 232 on the substrate 26 with different criticaldimensions.

[0156] In this embodiment, gray-tone regions of various transmissivitiesare used. The gray-tone regions are positioned in locationscorresponding to those features that are to be preserved in the trimexposure. Moreover, the transmissivity of each region may be between oneand zero, inclusive.

[0157] In a further embodiment of the present invention, interferometriclithography is used to pattern the fine features and a second trimexposure with gray-toning is used to locally vary the criticaldimension. In this embodiment, the interference nulls produced byinterferometry are used to form the fine features.

[0158] In a still further embodiment of the present invention, animprint process is used to pattern the fine features and a second trimexposure with gray-toning is used to locally vary the critical dimensionor to customized the substrate being formed by the imprinting of amaster template of fine features. In this embodiment, a pattern on asubstrate is imprinted using a master template. Thereafter, thesubstrate is exposed so as to change or customize the substrate. In apreferred embodiment of the above described imprint process, thesubstrate is photolithographically exposed.

[0159] The present invention may also use a trim mask when exposing theimprinted substrate wherein the trim mask may be a gray-tone mask. Thefeatures on the gray-tone mask may be regions of varioustransmissivities, pixellated regions, or sub-resolution features.

[0160] It is further noted that the sub-resolution features are featuresthat are not resolvable for a particular configuration of an exposuresystem. Moreover, according to the concepts of the present invention, anexposure system's parameters are separately optimized for each of thedouble exposure processes. The partial coherence of an exposure systemcan also be detuned, thereby allowing for larger sub-resolution featureson the mask. The numerical aperture of an exposure system can also bedecreased, thereby allowing for larger sub-resolution features on themask.

[0161] Furthermore, the combined exposure dose locally causes a reactionin the resist layer, and the reaction in the resist layer is dependenton the combined exposure dose. The reaction in the resist layer is usedto form features in the resist layer, and each feature in the resistlayer may have a desired critical dimension. The critical dimension ofeach feature may be determined by the design of the gray-tone mask.

[0162] The features in the resist layer may correspond to transistorgates, interconnect features, contact features, via features, orisolation features. The resist layer may be a positive or negativeresist layer.

[0163] In a still further embodiment of the present invention, a mask isdesigned in which a primary exposure is assumed. The mask is designed byplacing gray-tone features on a layout of the mask to locally adjust anexposure dose in regions corresponding to features defined in theprimary exposure and placing other features on the layout of the mask.The features defined in the primary exposure may be partially defined.The primary exposure may use a phase shift mask. The local adjustment ofthe exposure dose may result in a local adjustment of criticaldimension. The phase shift mask of the primary exposure may includegray-tone features. The gray-tone features may be formed by pixellationand have regions with different transmissivities wherein thetransmissivity of each region is between one and zero, inclusive.Moreover, the gray-tone features may correspond to regions withsub-resolution features, which when exposed produce regions of varyingintensity at a substrate plane. The sub-resolution features may befeatures that are not resolvable for a particular configuration of anexposure system.

[0164] Another aspect of the present invention is the actual designingof the features on a gray-tone mask. In this-embodiment, a computeraided design software package is used to design the mask. A userspecifies; using a user input device such as a mouse, keyboard, touchpad, touch screen, or microphone or the user may specify by using orinputting a data file or data stream containing the relevant location,dimensional, relational, and/or geometric data; the geometricproperties; such as location, critical dimension, orientation, a spatialrepresentation in two dimensions, a spatial representation in threedimensions, width dimensions length dimensions, thickness or heightdimensions, sidewall angles, curvatures, tapers, or any otherdimensional, relational, or geometric property of a substrate feature;of each desired substrate feature or features. From the user specifiedgeometric properties of the desired substrate feature or features; themask features of a gray-tone mask are automatically determined. Thesedetermined mask features could then be formed in an actual gray-tonemask.

[0165] These determined mask features could be mask features associatedwith a phase shift mask or a binary mask. The determined mask featurescan also be gray-tone features, regions of varying transmissivity,pixellated regions, or sub-resolution features.

[0166] The mask features may be determined in an automated manner usingan optical simulation engine or using geometry-based rules.

[0167] It is noted that the computer aided design software package canalso contemplate the use of a template feature placement whendetermining the mask features. In summary, the present invention isdirected to an imaging approach that overcomes the limitations of theconventional techniques, and confers a number of advantages. Opticalproximity effects are effectively eliminated, thereby eliminating theneed for costly and complex optical proximity correction. Spatialfrequency effects such as corner rounding and line-end shortening areminimized as well through the use of multiple, substantially independentoptical exposures. These attributes allow for maximum pattern densityand flexibility for applications.

[0168] Moreover, the present invention is also directed to a method ofimaging multiple critical dimension features in multiple exposurelithography. The use of chrome regulator features is not required bysome embodiments of the present invention. Therefore, some embodimentsof the present invention permit the imaging of multiple criticaldimension features using either chromeless phase shift orinterferometric lithography by using gray-tone features on either thephase-shift or trim mask to locally control the critical dimension. Thegray-tone features transmit zero order light, but contain no modulationinformation.

[0169] In accordance with the concepts of the present invention,superior dimension control is provided through pitch. The entirefine-line imaging process can be optimized for “dense-only” featuresthus permitting densities otherwise unachievable. Mask fabrication andinspection is simplified through the use of potentially re-usableregular feature arrays, thus lowering cost. The circuit design cycle isalso shortened since library cells may be pre-characterized in a mannerthat is independent of cell placement.

[0170] The present invention forms dense and regular features and removethose that are undesirable, thereby avoiding simultaneously creatingdense and isolated features. This is useful for patterns involvingsub-wavelength feature sizes and narrow pitch values. In a preferredembodiment, the reticle is patterned to provide a regular array ofdensely populated narrow features on the wafer that are consistent insize and shape. A trimming process is then used to remove undesiredfeatures.

[0171] The present invention also avoids circuit density penaltiesresulting from constraint of fine features in which tight size toleranceis essential to one orientation and grid position offset by adequatelyproviding for the geometric union of exposures at the wafer plane andallowing certain exposures or sets of exposures to have imaging that issubstantially independent from that of previous exposures or sets ofexposures.

[0172] As noted before, the above description refers to gate patternsmade using positive photoresist. The same process with negativephotoresist will make “trench” patterns useful for damascene gateapplications for example. The preferred embodiment also utilizes aphase-shift mask to produce the dense features. These dense regularfeatures could also be produced with other, non-optical mask projectionmethods such as laser interferometry or imprint/embossing methods.

[0173] One preferred embodiment describes the layout and fabrication ofnarrow line features that could represent transistor gate features.Alternate embodiments of the same method can be used to produce randomcontact hole or pillar arrays. These methods could form a densecontact/pillar pattern by multiple exposures of dense feature patternswith different offsets and/or orientations. Alternatively, dense contactpatterns can be formed by an attenuated phase-shift mask andsubsequently trimmed to form a random array. The resist's tonedetermines whether contacts or pillars are formed in this way.

[0174] Another preferred embodiment describes a dual resist process.Alternative embodiments could utilize either a hardmask process, inwhich an additional masking layer, typically silicon nitride or silicondioxide, is formed on the wafer prior to resist coating, this layerbeing defined and etched to form a mask that can be used in processingof the underlying substrate; or double bilayer resist processes in whichtwo layers of resist are formed, such that patterns formed in the toplayer of resist mask correspond to removal of the bottom layer of resistand a pattern formed in the bottom layer of resist corresponds to thegeometric union of multiple exposures.

[0175] Any multiple layer resist process, in which the photo-inducedreaction of a particular layer is substantially independent of thephoto-induced reaction of another layer, can be utilized with thepresent invention. Also, any resist process in which multiplesubstantially independent photo-induced reactions occur within the samelayer of resist can be used.

[0176] Lastly, the present invention provides a method to locally varythe critical dimension through the use of gray-tone mask features eitheron the fine feature or trim exposure masks in a multiple exposurelithography process.

[0177] Although the various concepts of the present invention have beendescribed above primarily using double exposure photolithographyexamples, the various described concepts of the present invention arealso equally applicable to interferometric processes or nanoimprintprocesses.

[0178] While the present invention has been particularly shown anddescribed with references to preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made herein without departing from the spirit and scope ofthe invention as defined by the appended claims.

What is claimed is:
 1. A method of forming a pattern on a substrate,comprising: (a) forming patterns on a substrate; and (b) exposing thesubstrate using a mask containing gray-tone features.
 2. The method asclaimed in claim 1, wherein said (a) consists of photolithographicallyexposing the substrate.
 3. The method as claimed in claim 1, whereinsaid (a) consists of interferometrically exposing the substrate.
 4. Themethod as claimed in claim 1, wherein said (a) consists of patternformation using an imprint method.
 5. The method as claimed in claim 1,wherein said (a) is performed using a photomask.
 6. The method asclaimed in claim 5, wherein the photomask is a phase shift mask.
 7. Themethod as claimed in claim 5, wherein the photomask has regions withdifferent transmissivities.
 8. The method as claimed in claim 7, whereinthe transmissivity of each region is between one and zero, inclusive. 9.The method as claimed in claim 6, wherein the phase shift mask hasregions that shift the relative phase of the transmitted light bydifferent phase angles.
 10. The method as claimed in claim 9, whereinthe relative phase angle corresponding to each region is between +180degrees and −180 degrees, inclusive.
 11. The method as claimed in claim5, wherein the photomask is a binary photomask.
 12. The method asclaimed in claim 1, wherein said (b) consists of photolithographicallyexposing the substrate using a gray-tone mask.
 13. The method as claimedin claim 1, wherein the mask containing gray-tone features is aphotomask.
 14. The method as claimed in claim 1, wherein the maskcontaining gray-tone features has regions with differenttransmissivities.
 15. The method as claimed in claim 14, wherein thetransmissivity of each region is between one and zero, inclusive. 16.The method as claimed in claim 1, wherein the gray-tone features on themask containing gray-tone features are formed using pixellation.
 17. Themethod as claimed in claim 1, wherein the gray-tone features on the maskcontaining gray-tone features are sub-resolution features, which whenexposed produce regions of varying intensity at the substrate plane. 18.The method as claimed in claim 17, wherein the sub-resolution featuresare features that are not resolvable for a particular configuration ofan exposure system.
 19. The method as claimed in claim 1, wherein anexposure system's parameters are separately optimized for said (a) andsaid (b).
 20. The method as claimed in claim 17, wherein a partialcoherence of an exposure system is detuned, thereby allowing for largersub-resolution features on the mask.
 21. The method as claimed in claim17, wherein a numerical aperture of an exposure system is decreased,thereby allowing for larger sub-resolution features on the mask.
 22. Themethod as claimed in claim 1, wherein the substrate includes a resistlayer.
 23. The method as claimed in claim 22, wherein at a substrateplane, the combined exposure dose corresponding to said (a) and said (b)locally causes a reaction in the resist layer.
 24. The method as claimedin claim 23, wherein the reaction in the resist layer is dependent onthe combined exposure dose.
 25. The method as claimed in claim 24,wherein the reaction in the resist layer is used to form features in theresist layer.
 26. The method as claimed in claim 25, wherein eachfeature in the resist layer has a desired critical dimension.
 27. Themethod as claimed in claim 25, wherein features of multiple criticaldimensions are desired on the substrate.
 28. The method as claimed inclaim 25, wherein a critical dimension of each feature is determined bythe design of the gray-tone mask.
 29. The method as claimed in claim 25,wherein features in the resist layer correspond to transistor gates. 30.The method as claimed in claim 25, wherein features in the resist layercorrespond to interconnect features.
 31. The method as claimed in claim25, wherein features in the resist layer correspond to contact features.32. The method as claimed in claim 25, wherein features in the resistlayer correspond to via features.
 33. The method as claimed in claim 25,wherein features in the resist layer correspond to isolation features.34. The method as claimed in claim 22, wherein the resist layer is apositive resist layer.
 35. The method as claimed in claim 22, whereinthe resist layer is a negative resist layer.
 36. The method as claimedin claim 5, wherein the photomask includes regular features.
 37. Themethod as claimed in claim 36, wherein the photomask includes regulardense features.
 38. The method as claimed in claim 5, wherein thephotomask includes locally regular features.
 39. The method as claimedin claim 38, wherein the photomask includes locally regular densefeatures.
 40. A method of designing a mask in which a primary exposureis assumed, comprising: (a) placing gray-tone features on a layout ofthe mask to locally adjust an exposure dose in regions corresponding tofeatures defined in the primary exposure; and (b) placing other featureson the layout of the mask.
 41. The method as claimed in claim 40,wherein features defined in the primary exposure are partially defined.42. The method as claimed in claim 40, wherein the primary exposure usesa phase shift mask.
 43. The method as claimed in claim 40, whereinlocally adjusting the exposure dose results in a local adjustment ofcritical dimension.
 44. The method as claimed in claim 42, wherein thephase shift mask includes gray-tone features.
 45. The method as claimedin claim 44, wherein the gray-tone features are formed by pixellation.46. The method as claimed in claim 44, wherein the gray-tone featuresare regions with different transmissivities.
 47. The method as claimedin claim 46, wherein the transmissivity of each region is between oneand zero, inclusive.
 48. The method as claimed in claim 44, wherein thegray-tone features correspond to regions with sub-resolution features,which when exposed produce regions of varying intensity at a substrateplane.
 49. The method as claimed in claim 48, wherein the sub-resolutionfeatures are features that are not resolvable for a particularconfiguration of an exposure system.
 50. A trim mask comprisinggray-tone features.
 51. The trim mask as claimed in claim 50, whereinthe trim mask is designed for use in a multiple exposure lithographymethod.
 52. The trim mask as claimed in claim 50, wherein a position ofsaid gray-tone features on the trim mask is a function of the positionof fine features on a separate fine feature definition mask.
 53. Thetrim mask as claimed in claim 50, wherein said gray-tone features areproduced by pixellation.
 54. The trim mask as claimed in claim 50,wherein said gray-tone features are regions with differenttransmissivities.
 55. The trim mask as claimed in claim 54, wherein thetransmissivity of each region is between one and zero, inclusive. 56.The trim mask as claimed in claim 50, wherein said gray-tone featurescorrespond to regions with sub-resolution features, which when exposedproduce regions of varying intensity at a substrate plane.
 57. The trimmask as claimed in claim 56, wherein the sub-resolution features arefeatures that are not resolvable for a particular configuration of anexposure system.
 58. The method as claimed in claim 6, wherein the phaseshift mask is a strong phase shift mask.
 59. The method as claimed inclaim 6, wherein the phase shift mask is a weak phase shift mask. 60.The method as claimed in claim 42, wherein the phase shift mask is astrong phase shift mask.
 61. The method as claimed in claim 42, whereinthe phase shift mask is a weak phase shift mask.
 62. The method asclaimed in claim 1, wherein said (a) uses an exposure dose above aresist exposure threshold.
 63. The method as claimed in claim 1, whereinsaid (a) uses an exposure dose below a resist exposure threshold. 64.The method as claimed in claim 1, wherein said (b) uses an exposure doseabove a resist exposure threshold.
 65. The method as claimed in claim 1,wherein said (b) uses an exposure dose below a resist exposurethreshold.
 66. A method of forming a feature having a critical dimensionon a substrate, comprising: (a) exposing the substrate using a trim maskcontaining gray-tone features.
 67. The method as claimed in claim 66,wherein said (a) consists of photolithographically exposing thesubstrate using a gray-tone mask.
 68. The method as claimed in claim 66,wherein the trim mask containing gray-tone features is a photomask. 69.The method as claimed in claim 66, wherein the trim mask containinggray-tone features has regions with different transmissivities.
 70. Themethod as claimed in claim 69, wherein the transmissivity of each regionis between one and zero, inclusive.
 71. The method as claimed in claim66, wherein the gray-tone features on the trim mask containing gray-tonefeatures are formed using pixellation.
 72. The method as claimed inclaim 66, wherein the gray-tone features on the trim mask containinggray-tone features are sub-resolution features, which when exposedproduce regions of varying intensity at the substrate plane.
 73. Themethod as claimed in claim 72, wherein the sub-resolution features arefeatures that are not resolvable for a particular configuration of anexposure system.
 74. A mask set for a process for providing patterns ona substrate comprising: a fine feature mask containing a pattern ofdense features; and a trim mask containing gray-tone features to producemultiple trimmed patterns of fine features.
 75. The mask set as claimedin claim 74, further comprising: an additional mask or set of masks toprovide additional features with the imaging substantially independentof the previous exposures.
 76. The mask set as claimed in claim 74,wherein said fine feature mask contains a pattern of regular densefeatures.
 77. The mask set as claimed in claim 74, wherein said finefeature mask contains a pattern of dense features of a predeterminedpitch and critical dimension.
 78. The mask set as claimed in claim 74,wherein said gray-tone features on said trim mask correspond totransistor gates located on a regular pattern.
 79. The mask set asclaimed in claim 74, wherein said gray-tone features on said trim maskcorrespond to hole or pillar features located on a regular pattern. 80.The mask set as claimed in claim 74, wherein said gray-tone features onthe trim mask correspond to interconnect segments located on a regularpattern.
 81. The mask set as claimed in claim 74, wherein said gray-tonefeatures on said trim mask correspond to transistor gates located on aregular pattern.
 82. The mask set as claimed in claim 74, wherein saidgray-tone features on said trim mask correspond to hole or pillarfeatures located on an optically dense feature pattern.
 83. The mask setas claimed in claim 74, wherein said gray-tone features on the trim maskcorrespond to interconnect segments located on an optically densefeature pattern.
 84. The mask set as claimed in claim 75, wherein saidadditional mask or set of masks includes a fine feature mask containinga pattern of dense features of a predetermined pitch and criticaldimension and a trim mask containing gray-tone features to producemultiple trimmed patterns of fine features.
 85. The mask set as claimedin claim 74, wherein said fine feature mask is replaced by aninterferometric pattern.
 86. A method of forming a random contact arrayon a substrate comprising: (a) exposing the substrate to provide apattern of dense contact features of a predetermined pitch and criticaldimension; and (b) exposing the substrate with a trim mask containinggray-tone features to provide multiple trimmed patterns on thesubstrate, the trimmed patterns including both densely populated andsparsely populated regions of features, the critical dimension of thefeatures in the densely populated regions and sparsely populated regionsbeing substantially independent of feature density.
 87. The method asclaimed in claim 86, wherein the substrate is photolithographicallyexposed to provide a pattern of regular contact features of apredetermined pitch and critical dimension.
 88. The method as claimed inclaim 86, wherein the substrate is photolithographically exposed toprovide multiple trimmed patterns on the substrate, the trimmed patternsincluding both densely populated and sparsely populated regions offeatures.
 89. The method as claimed in claim 86, wherein the pattern ofregular dense contact features is a pattern of regular dense holes. 90.The method as claimed in claim 86, wherein the pattern of regular densecontact features is a pattern of regular dense pillars.
 91. A mask setfor a process for providing patterns on a substrate comprising: a finefeature mask; and a trim mask containing gray-tone features to producemultiple trimmed patterns of fine features.
 92. The mask set as claimedin claim 91, further comprising: an additional mask or set of masks toprovide additional features with the imaging substantially independentof the previous exposures.
 93. The mask set as claimed in claim 91,wherein said gray-tone features on said trim mask correspond totransistor gates located on a regular pattern.
 94. The mask set asclaimed in claim 91, wherein said gray-tone features on said trim maskcorrespond to hole or pillar features located on a regular pattern. 95.The mask set as claimed in claim 91, wherein said gray-tone features onthe trim mask correspond to interconnect segments located on a regularpattern.
 96. The mask set as claimed in claim 91, wherein said gray-tonefeatures on said trim mask correspond to transistor gates located on aregular pattern.
 97. The mask set as claimed in claim 91, wherein saidgray-tone features on said trim mask correspond to hole or pillarfeatures located on an optically dense feature pattern.
 98. The mask setas claimed in claim 91, wherein said gray-tone features on the trim maskcorrespond to interconnect segments located on an optically densefeature pattern.
 99. The mask set as claimed in claim 91, wherein saidfine feature mask is replaced by an interferometric pattern.
 100. Acomputer aided design method for designing a mask, comprising: (a)specifying, through an input of a user, a geometric property of adesired substrate feature; and (b) determining automatically, based uponthe user specified geometric property of the desired substrate feature,mask features for a gray-tone mask.
 101. The computer aided designmethod as claimed in claim 100, wherein the determined mask featurescorrespond to mask features on a phase shift mask.
 102. The computeraided design method as claimed in claim 100, wherein the determined maskfeatures correspond to mask features on a binary mask.
 103. The computeraided design method as claimed in claim 101, wherein some of thedetermined mask features on the phase shift mask are gray-tone features.104. The computer aided design method as claimed in claim 103, whereinthe gray-tone features are regions of varying transmissivity.
 105. Thecomputer aided design method as claimed in claim 103, wherein thegray-tone features are pixellated regions.
 106. The computer aideddesign method as claimed in claim 103, wherein the gray-tone featuresare sub-resolution features.
 107. The computer aided design method asclaimed in claim 100, wherein the mask features are regions of varioustransmissivities.
 108. The computer aided design method as claimed inclaim 100, wherein the mask features are pixellated regions.
 109. Thecomputer aided design method as claimed in claim 100, wherein the maskfeatures are sub-resolution features.
 110. The computer aided designmethod as claimed in claim 100, wherein the mask features are derived inan automated manner using an optical simulation engine.
 111. Thecomputer aided design method as claimed in claim 100, wherein the maskfeatures are derived in an automated manner using geometry-based rules.112. The computer aided design method as claimed in claim 100, wherein atemplate is utilized in feature placement.
 113. The computer aideddesign method as claimed in claim 103, wherein the mask features arederived in an automated manner using an optical simulation engine. 114.The computer aided design method as claimed in claim 103, wherein themask features are derived in an automated manner using geometry-basedrules.
 115. The computer aided design method as claimed in claim 100,wherein the desired geometric property is a critical dimension of thedesired substrate feature.
 116. The computer aided design method asclaimed in claim 100, wherein the desired geometric property is alocation of the desired substrate feature.
 117. The computer aideddesign method as claimed in claim 100, wherein the desired geometricproperty is orientation of the desired substrate feature.
 118. Thecomputer aided design method as claimed in claim 100, wherein thedesired geometric property is a spatial representation in two dimensionsof the desired substrate feature.
 119. The computer aided design methodas claimed in claim 100, wherein the desired geometric property is aspatial representation in three dimensions of the desired substratefeature.
 120. The computer aided design method as claimed in claim 100,wherein the desired geometric property is a width dimension of thedesired substrate feature.
 121. The computer aided design method asclaimed in claim 100, wherein the desired geometric property is a lengthdimension of the desired substrate feature.
 122. The computer aideddesign method as claimed in claim 100, wherein the desired geometricproperty is a thickness dimension of the desired substrate feature. 123.The computer aided design method as claimed in claim 100, wherein thedesired geometric property is a sidewall angle of the desired substratefeature.
 124. The computer aided design method as claimed in claim 100,wherein the desired geometric property is a curvature of the desiredsubstrate feature.
 125. The computer aided design method as claimed inclaim 100, wherein the desired geometric property is a taper of thedesired substrate feature.
 126. The computer aided design method asclaimed in claim 100, wherein the desired geometric property is input bythe user using an input/output device.
 127. The computer aided designmethod as claimed in claim 100, wherein the desired geometric propertyis input by the user using a data file.
 128. The computer aided designmethod as claimed in claim 100, wherein the desired geometric propertyis input by the user using a data stream.
 129. A method of forming apattern on a substrate, comprising: (a) imprinting a pattern on asubstrate; and (b) exposing the substrate to change the imprintedpattern.
 130. The method as claimed in claim 129, wherein (b)photolithographically exposes the substrate.
 131. The method as claimedin claim 130, wherein (b) exposes the substrate using a trim mask. 132.The method as claimed in claim 131, wherein the trim mask is a gray-tonemask.
 133. The method as claimed in claim 132, wherein features on thegray-tone mask are regions of various transmissivities.
 134. The methodas claimed in claim 132, wherein features on the gray-tone mask arepixellated regions.
 135. The method as claimed in claim 132, whereinfeatures on the gray-tone mask are sub-resolution features.